BEIRUT, Jan 29 (Reuters) - The United Nations has receivedpromises of major donations at this week's $1.5 billion aidconference for millions of Syrians affected by nearly two yearsof conflict, a senior U.N. official said on Tuesday.

Wednesday's pledging conference in Kuwait will seek $1billion of aid for Syria's neighbours sheltering 700,000registered refugees, and another $500 million to bankrollhumanitarian work for 4 million Syrians inside their country.

So far, the United Nations has received pledges coveringjust 18 percent of the target, unveiled last month as the scaleof Syria's humanitarian crisis escalated sharply, and which aimsto fund operations for the first half of this year.

"We have every reason to be optimistic that there will avery good presence and new pledges that will be coming up atthis conference," said Robert Watkins, U.N. humanitariancoordinator in Lebanon, which has seen the biggest influx ofrefugees from the Syrian bloodshed.

"We have received preliminary pledges from a number ofimportant donor countries that they will be making announcementsof large donations."

U.S. President Barack Obama announced an additional $155million, bringing the total U.S. humanitarian aid to the Syriancrisis to some $365 million, the State Department said.

Watkins said the fact that the conference was being held inthe Gulf state of Kuwait could encourage other wealthy Gulf Arabstates, who have led regional opposition to President Basharal-Assad, to support the international aid effort.

Many Gulf states have sent assistance, but aid workers inthe region say their efforts have been haphazard and rarelycoordinated with other aid agencies, hampering their ability toplan a sustained relief programme.

"It's important that we widen the range of donors thatprovide assistance," Watkins said.

OPPOSITION CRITICISM

Syria's main opposition coalition has criticised the U.N.appeal and its arrangements for distributing aid inside Syria,saying the organisation has effectively ceded control to theSyrian government and failed to deliver all but a bare minimumof aid to areas controlled by Assad's opponents.

But Watkins said the Syrian government did not influence theaid distribution, although there were some areas of the countrywhich could not be reached because of the violence.

"While we are not able to reach all of the people in need inSyria - because there are 4 million people who need assistanceinside the country - we have been able to feed up to 1.5 millionpeople," Watkins said.

"Of those 1.5 million people, 49 percent are in areas whichare either under the control of the opposition or in contestedareas," he said, adding that the main U.N. aid distributionpartner in Syria - the Syrian Arab Red Crescent (SARC) - was"neutral and impartial".

The opposition says the top members of the SARC are close toAssad's government, and the organisation cannot operateeven-handedly. But activists say its workers on the ground havecome under fire from both sides in the conflict.

Syria's uprising began in March 2011 as a peaceful protestmovement, but rebels took up arms after the government crackeddown on the demonstrations.

The rebellion has since become a full-scale civil war.Insurgents have taken swathes of rural territory from governmentforces but have failed to capture major cities and towns. Morethan 60,000 people have died, the United Nations says.

The number of refugees fleeing to neighbouring countries hasjumped sharply in the last two months, passing the 500,000 markon Dec. 11 and growing by more than 200,000 since then.

On Monday, 3,000 Syrians fled into Jordan.

"We have seen an unrelenting flow of refugees across allborders. We are running double shifts to register people,"Sybella Wilkes, spokeswoman for the U.N. High Commissioner forRefugees (UNHCR), told Reuters in Geneva.

Jordan has 171,033 registered Syrian refugees, as well as51,729 who await processing, many of who fled fighting aroundthe southern Syrian town of Deraa this month. Lebanon has158,973 Syrian refugees, and 69,963 awaiting processing.

"We are trying to clear a backlog of people because thenumbers have gone up so dramatically (in Jordan and Lebanon),"Wilkes said.

Turkey has 163,161 Syrian refugees in its 15 camps whileIraq hosts 77,415, the UNHCR said. There are 14,375 in Egypt and5,417 registered across the rest of North Africa.